(a) Field of the Invention:
The present invention relates to an objective lens system consisting of a single lens component and to be used in imaging optical systems having relatively high image heights such as objective lens systems for microscopes.
(b) Description of the prior art:
Objective lens systems each of which consists of a single lens component are widely used in the information recording-readout optical systems for optical disks since the objective lens systems have the simplest composition and are compact. The information recording-readout optical systems for optical disks require no correction of chromatic aberration since the systems use lasers having a single wavelength as a light source. For this reason, each of the objective lens systems used in this type of optical systems consists mostly of a single lens element. In the recent days, however, there have been developed optical systems which perform recording and readout of information by using a plural number of laser sources emitting lights of wavelengths different from one another. Accordingly, there are known objective lens systems each of which consists of a cemented doublet for use in these optical systems. For example, there is known the objective lens system disclosed by Japanese Unexamined Published patent application No. 3110/61. This objective lens system consists of a cemented doublet consisting of a negative meniscus lens element and a biconvex lens element arranged in the order from the object side (on the side of the optical disk), is so designed as to favorably correct chromatic aberration by selecting adequate Abbe's numbers for the two lens elements, and is so adopted as to correct the spherical aberration and the sine condition within a range up to a large numerical aperture by using aspherical surfaces on both the lens elements.
In the objective lens systems used in the information recording-readout optical systems, however, the aberrations are corrected only within a very narrow range in the vicinity of the optical axis. These objective lens systems are low in the offaxial performance thereof, and cannot practically be used as the objective lens systems for microscopes and the similar instruments which must form images of objects having sizes of certain degrees.